Measuring the binding affinity between materials, molecules, and cells is key to a broad spectrum of industries, including material development, semiconductor production, bioanalytical assays, biomedical diagnostics, and drug discovery. With the emergence of solid state array-based bioanalytical and genetic diagnostic instruments and related equipment, new methods for cost effective screening of a large number of reactions in a miniaturized solid state form have become increasingly desirable. The favored approach to date is to monitor changes in optical properties, usually fluorescence, when a known, fluorescently labeled molecule interacts with a known molecular species at a specific address in a molecular array. These apparatuses and methods, however, often impose stereochemical constraints by the addition of reporter systems to the molecules used to interrogate the molecular array. Thus, label free, direct interrogation of molecular binding events using a micromechanical reporter is of obvious utility. More sophisticated and robust instrumentation for the creation of these molecular arrays is therefore desirable.
One method for the direct detection of molecular interaction events is the scanning probe microscope. One type of scanning probe microscope is the atomic force microscope (“AFM”). In the AFM, a sharp tip is situated at the end of a flexible cantilever and scanned over a sample surface. While scanning, the cantilever is deflected by the net sum of the attractive and repulsive forces between the tip and sample. If the spring constant of the cantilever is known, the net interaction force can be accurately determined from the deflection of the cantilever. The deflection of the cantilever is usually measured by the reflection of a focused laser beam from the back of the cantilever onto a split photodiode, constituting an “optical lever” or “beam deflection” mechanism. Other methods for the detection of cantilever deflection include interferometry and piezoelectric strain gauges.
The first AFMs recorded only the vertical displacements of the cantilever. More recent methods involve resonating the tip and allowing only transient contact, or in some cases no contact at all, between it and the sample. Plots of tip displacement or resonance changes as it traverses a sample surface are used to generate topographic images. Such images have revealed the three dimensional structure of a wide variety of sample types including material, chemical, and biological specimens. Some examples of the latter include DNA, proteins, chromatin, chromosomes, ion channels, and even living cells.
In addition to its imaging capabilities, the AFM can make extremely fine force measurements. The AFM can directly sense and measure forces in the microNewton (10−6) to picoNewton (10−12) range. Thus, the AFM can measure forces between molecular pairs, and even within single molecules. Moreover, the AFM can measure a wide variety of other forces and phenomena, such as magnetic fields, thermal gradients and viscoelasticity. This ability can be exploited to map force fields on a sample surface, and reveal with high resolution the location and magnitude of these fields, as in, for example, localizing complexes of interest located on a specific surface. To make molecular force measurements, the AFM probe may be functionalized with a molecule of interest.
Construction of molecular arrays on a solid support for use in an AFM is typically carried out by processes that can be divided into two general classes: in situ and ex situ, the latter including a mechanical deposition step to actually place the sample on the deposition surface. In situ synthesis methods and apparatuses may involve photochemical synthesis of nucleic acid or short peptides to define the spatial addresses on a silicon or a glass surface. These methods maybe limited by the wavelength of light used for masking and the synthetic procedure. Furthermore, this procedure may also be limited by cost. A need therefore exists for a dedicated apparatus for the creation of molecular arrays that may create the array in a quick and efficient manner.
An example of an ex situ method followed by the mechanical deposition on the surface may be illustrated by the “dip pen” method. The sample material is prepared in advance and then the dip pen is used to place the sample on the deposition surface. It has been shown that a dip-pen method may be used to draw a submicron molecular line or spot using an alkanethiolate monolayer utilizing a standard AFM to control the dip pen. Other prior art instruments may utilize a pin tool which is dipped in a solution containing the sample material. The pin tool then has a drop of solution on it, which is then placed on the deposition surface. This method, however, does not allow the creation of extremely small deposition domains. Up until this time, AFMs have been utilized for drawing sub-micron molecular lines or creating the molecular spots. AFMs, however, are not optimal for creating arrays because they lack features, such as a sub-micron precision sample stage under computer control, precise optical access for sample registration, and unencumbered access to the software code used to control the tip motion. Furthermore, commercial AFM configurations are not amenable to the rapid deposition of large numbers of different molecular species. Finally, AFMs are designed for multiple tasks, not as a dedicated sample deposition instrument, and are therefore more expensive than is required for a dedicated arrayer. Still other features may also be desirable in a dedicated deposition instrument and not included with an AFM. A need therefore exists for an instrument that is dedicated to the creation of arrays comprised of deposition domains.
A need exists for a commercially practical deposition instrument that can be utilized to create a molecular deposition array that includes sub-micron deposition domains. This instrument may incorporate precise optical features for sample registration and may be controlled utilizing a computer control so that user defined array patterns and sizes may be created. It may be particularly advantageous if this instrument can operate autonomously in a high throughput format.